Nearly half of the increases in ocean temperature between 1865-2015 occurred in just the past 20 years, a rate which is steadily getting quicker, a new study published Monday reveals.

Deep underwater, below 700 meters, the ocean holds 35 percent of the world’s heat associated with greenhouse gases—an increase from the 20 percent it had absorbed just two decades ago, according to the study, published in the journalNature Climate Change. Little is known about temperature measurement at that depth, which is partially what makes the findings so distressing, the researchers said.

“In recent decades the ocean has continued to warm substantially, and with time the warming signal is reaching deeper into the ocean,” said lead author Peter Gleckler, a scientist with Lawrence Livermore National Laboratory (LLNL).